I have the latest version of MarketPress Installed in a Multi Site environment.
I am using SMThemes for all the sites. When I check out nothing displays in the shopping cart page. Example site is http://www.kellynolfmusic.com
PayPal Express is setup with all the API information. I am sure it is something in the SMTheme because it will work with the Twenty 11 WorPress Theme activated. I receive the same result with all the other SMThemes available to my clients.